In order to prevent illegal duplication of a document and to recognize that a document is a printed document whose duplication is prohibited, the following technologies are disclosed. As documents which are prohibited from being duplicated, there are certificates such as a resident card and a full copy of a family register, other important documents, a bank note, and securities such as a share, a bond.
For example, in Patent Document 1, an image processing apparatus is disclosed. The image processing apparatus includes a combining unit that combines an image of an object with an image pattern indicating that the image of the object is prohibited from being duplicated and an output unit which outputs data of the combined image to a predetermined output destination.
In addition, in Patent Document 2, another image processing apparatus is disclosed. The image processing apparatus includes an input unit for inputting color image data, a memorizing unit for storing identification information of an apparatus, and an output unit for outputting a color image shown by the color image data input by the input unit on a recording medium by using plural visible color materials. The output unit adds a specific pattern showing the identification information of the apparatus stored in the memorizing unit by using a color material which is difficult for human eyes to distinguish from of the plural visible color materials.
Further, in Patent Document 3, an electronic watermark inserting method, which inserts information into a digital image by using a frequency formation method (specifically, a Fourier transformation method), is disclosed. In Patent Document 4, a method is disclosed in which digital information readable by a machine is embedded in a half-tone image.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2004-260341    [Patent Document 2] Japanese Patent No. 2614369    [Patent Document 3] Japanese Patent No. 3522056    [Patent Document 4] Japanese Laid-Open Patent Application No. 11-313206
As another technology, paper called counterfeit preventing paper, in which a special printing is performed on the paper beforehand, is used. On the paper, a mark which prohibits duplication is printed with a concentration reproducible by a duplicating apparatus. The mark is, for example, “duplication prohibited”. In addition, the background is printed with a concentration by which the background is hardly reproduced. The concentration of the mark part and the background part is determined to be hardly distinguishable by human eyes. The mark part and the background part formed other than in the document region are called “tint blocks”.
The tint block is embedded in the paper so that information of a document can be hardly damaged, and must be a pattern from which the document can be distinguished as a duplicated one when the document is duplicated. In an electro-photographic system using in a printer such as a laser printer, printing of a tint block pattern has been realized.
In the electronic watermark inserting method described in Patent Document 3, in the frequency transformation process, generally, a large amount of data must be processes and it is very difficult to realize the method. In addition, in the method in which information is embedded in a half-tone image described in Patent Document 4, information can be embedded in a print (half-tone) image. However, the method cannot be applied to a binary image such as a document image for forming a half-tone image. In order to solve these problems, as described in Patent Document 1, the image data are output by combining an image of an object with an image pattern indicating that it is prohibited to duplicate the image of the object. Or as described in Patent Document 2, a special pattern showing identification information of the apparatus is added.
However, as an image forming apparatus, recently, in addition to an apparatus using the electro-photographic system, an image forming apparatus of an inkjet recording system has been widely used in which inkjet recording system a liquid ejecting head for ejecting ink droplets (recording liquid) is used as a recording head. In the inkjet recording apparatus, a liquid ejecting head providing a pressure generating unit which applies pressure to ink in an ink chamber to which a nozzle for ejecting ink droplets is connected is used as a recording head, a force is given to ink in the liquid chamber by driving the pressure generating unit of the liquid ejecting head corresponding to image information, and the ink droplets are ejected from the nozzle. With this, an image is formed on a recording medium such as a paper and a film by attaching the ink droplets onto the recording medium.
There are differences in a dot position and a dot size which can be formed and an expansion of the dot on the recording medium between printing in the inkjet recording system and printing in the electro-photographic system. That is, in the electro-photographic system, a dot can be formed on an approximately arbitrary position; however, in the inkjet recording system, dot positions which can be formed are restricted by the nozzle pitch of the recording head and a driving frequency. In addition, in the electro-photographic system, the dot does not expand into the recording medium because there is no penetrating toner transferred to the recording medium. However, in the inkjet recording system, the recording liquid penetrates into the recording medium after being ejected; consequently, the dot is expanded.
In a case where printing is performed by using an inkjet recording apparatus which forms an image by ejecting ink droplets, in Patent Document 1, it is difficult to output a pattern indicating that the document is prohibited from being duplicated, and in Patent Document 2, it is difficult to output a special pattern showing identification information of the apparatus.